When a joint, such as a shoulder, hip or knee, becomes impaired due to arthritis, disease or trauma, it is sometimes necessary to replace all or part of the joint with one or more prostheses to restore function. Four anatomical characteristics that relate to shoulders are humeral and glenoid inclination and humeral retroversion and glenoid version, all of which can change as disease in the shoulder joint progresses.
For purposes of this disclosure, humeral inclination is defined as the angle between the plane of the anatomical neck of the proximal humerus and the metaphyseal axis of the humerus. Glenoid inclination is defined in the coronal plane as the angle between a line identifying the inferior-superior axis of the glenoid and a line connecting the intersection of the scapular spine with the medial border of the scapula and the middle of the spinoglenoid notch. Further, humeral retroversion (or humeral torsion) is defined as the angle between the normal vector to a plane defined by the perimeter of the anatomical neck and the transepicondylar axis of the distal humerus. Finally, glenoid version is defined in the axillary view as the angle between a line identifying the anterior-posterior rim of the glenoid cavity and a line perpendicular to a line identifying the scapular blade. Correction and modification of both humeral and glenoid inclination and version changes in diseased shoulder joints are benefits of successful shoulder implant procedures.
Anatomically, the average humeral inclination is approximately 135°, but can range from about 120° to about 150°. Average humeral retroversion is approximately 30°, but can range from about −5° to about 60°. Glenoid inclination can range from about 80° to about 100°, whereas glenoid version can range from about 0° to about −14°. In order to optimize balancing of soft tissues and post-operative function, it is crucial to establish the proper inclination and version for each patient. Incorrect inclination and version can lead to problems including limited function, subsequent dislocations, and prosthetic component loosening and wear. Unfortunately, with existing implants and installation techniques, these critical parameters are chosen in a freehand manner.
In many cases, when an implant is used as treatment for an arthritic shoulder joint, it is necessary to remove or resect the diseased humeral head and prepare the proximal end of the humeral shaft to receive the stem, body, inclination set and head. It is important that the humeral head preparation be accurate so that the position of the implant, which is determined in part by the configuration of the humeral head, replicates the original anatomic position of the humeral head.
In less severe cases, the humeral head may be resurfaced by installing an implant that covers the humeral head after the humeral head has been resurfaced which removes abnormalities such as osteophytes, tuberositites, etc. In more severe cases, such as irreparable rotator cuff rupture, standard shoulder implants, as described above, would not provide sufficient pain-free joint stability or an adequate range of motion. Thus, an implant with a “reverse” joint has been developed where a base plate is attached to the glenoid fossa with screws after the glenoid fossa has been resurfaced. A head, sometimes referred to as a glenosphere, is attached to the glenoid base plate, as opposed to the humeral head. The humeral head is still resected and a stem is inserted downward into the intramedullary canal. A cup-shaped reverse body is attached to the proximal end of the stem, which receives a polymeric insert, receives the head or glenosphere that is fixed to the glenoid base plate.
The above procedures, while providing significant advancements, still suffer from certain drawbacks. For example, the most critical step in a humeral head resurfacing technique is placement of the initial guide wire, over which the humeral head resurfacing body slides. Unfortunately, the placement of the guidewire is performed using a freehand technique and an alignment tool. Contemporaneously, the surgeon estimates the desired inclination from the coronal (vertical) plane or transverse (horizontal) plane and maintains this desired inclination while the surgeon estimates a desired retroversion. Holding the head template in this exact position, the surgeon then drive drives the central guide wire or K-wire through the guide of the template using a drill or other suitable tool.
Thus, an optimal final positioning of the symmetrical implant on the asymmetrical humeral head is dependent on the surgeon's ability to visualize the desired orientation in three dimensions and the surgeon's ability to hold the head template steady while inserting the guide wire.
For traditional (non-reverse) shoulder implants, many surgeons use intramedullary cutting jigs or a freehand technique to resect the humeral head. For the more severe cases that require a reverse implant as described above, an intramedullary cutting jig is required to minimize the possibility of reaming with the incorrect retroversion, which could cause the screws that anchor the glenoid base plate to extend through or “break out” of the cortical bone structure of the glenohumeral joint or scapula. To avoid this problem, a complex intramedullary cutting jig is used